1. Field of the Invention
The present invention relates to a semiconductor process, and more especially, to an etching process with high selectivity to polysilicon over silicon oxide but without hardening to photoresist.
2. Description of the Prior Art
Etching process, which includes dry etching and wet etching, is used to remove some materials from a wafer. The wet etching is usually applied to isotropically strip an entire film or a residual film that has been undergone some other processes, by using etchant dissolved in a solution. The dry etching is generally applied to anisotropically remove a portion of a film to transfer a pattern from a mask, by using plasma gases as an etching source.
One of the important structures that needs for pattern transference is the gate electrode of a metal-oxide-semiconductor field effect transistor (MOSFET), which is typically used doped polysilicon as the material, and, of course, an anisotropic etching as plasma etching will be a suitable method to carry out this pattern transference. Considering about the selectivity between polysilicon and silicon dioxide used for gate oxide, the plasma source applied to anisotropically etch polysilicon can be chloride containing gases including Cl.sub.2, HCl, and SiCl.sub.4, and so on. Another preferred plasma source for polysilicon etching is HBr, which can provide a better selectivity to polysilicon over silicon dioxide, and is frequently used in high integration semiconductor process, with He and He/O.sub.2 mixing together.
In a polysilicon etching system using HBr, He and He/O.sub.2 as the etching gases, the process chamber is kept in a pressure greater than 30 mTorr. The flow rate of HBr, He and He/O.sub.2 are in the range of about 100-200 sccm, 100-200 sccm and 5-10 sccm respectively, with a total flow rate at about 200 to 300 sccm. These flow rates can be controlled by the pumping system and the valves deposed on the gas delivery lines. With such a process parameters choice, a high selectivity about to polysilicon over silicon dioxide can be achieve, and the gate oxide under the polysilicon gate electrode can substantially be prevented from the etching reaction. An ultra-thin gate oxide and a semiconductor MOSFET device with high integration can therefore be attended.
However, in above standard operating conditions used in polysilicon etching by HBr, the plasma gases will react with the material of the photoresist used for pattern. Materials of polymer are produced in this reaction as by-product. This by-product of polymer has a harder structure than the original photoresist material, and the photoresist is thus hardened. The hardened photoresist is difficult to be stripped, and needs a complex post treatment. Moreover, in extreme situation, the produced by-product of polymer can distribute on the reactive film and behave as hard mask to obstruct the proceeding of the etching. Therefore, it needs a proper method to deal with this problem of photoresist hardening.
In the conventional manner, the photoresist is stripped by a two-step process after patterning. In general, an photoresist ashing process, normally a dry etching, followed by a wet cleaning, can provide a complete removal for the ordinary photoresist. However, for the hardened photoresist, an additional photoresist ashing process is needed after the wet cleaning, because the original two steps can not efficiently clear all the hardened photoresist.
Alternatively, a hard mask such as nitride mask can be utilized to cover a portion of polysilicon film instead of the photoresist during the etching process to prevent the producing of polymer. By this way, the photoresist hardening is no long a problem, but the hard mask forming process is much complex than the photoresist mask. For instance, it takes a nitride deposition, a photolithography process, a nitride etching, and a photoresist stripping before polysilicon etching, and a nitride stripping after. In the mean time, the photoresist mask takes only photolithography process and photoresist stripping.
No matter adding a photoresist ashing process after the wet cleaning, or using a hard mask instead of the photoresist mask, it takes extra process to solve the photoresist-hardening problem, and will loss efficiency and cost a higher capital. Both of these two methods are not the most ideal way to solve the problem of photoresist hardening causing by the producing of polymer.